Advances in film forming technology have yielded improvements in disposable absorbent products such as disposable diapers, feminine hygiene products and the like. “Film” is a common term for thermoplastic polymer webs made from any variety of processes. The most common method of producing films is with an extrusion process.
Cast extrusion and blown extrusion are commonly known methods in the film producing industry. In a blown extrusion process, a circular die extrudes an inflated bubble of film that is cooled by cool air streams blown onto the bubble's perimeter by an air ring. The bubble is then flattened in a nip and subsequently slit into flat sheets that can then be reheat embossed or otherwise manipulated. Blown film can be used to create a roll of precursor film that may be fed into a reheat vacuum formed film (VFF) process. This method is taught by U.S. Pat. No. 4,151,240 to Lucas. Additionally, it is also known to use a precursor roll of cast film.
In a cast extrusion process, a flat web is extruded from a slot die. The flat web is subsequently cooled and set by a variety of chilling roller means. As an example, U.S. Pat. No. 4,456,570 to Thomas teaches a cast extrusion in a direct melt vacuum formed film (VFF) process. In a vacuum formed film process, a pressure differential is applied across a forming screen. In the case of a direct melt VFF process, a molten web is extruded onto a forming area of a forming screen. An example of a direct melt VFF process is taught by U.S. Pat. No. 4,456,570 to Thomas. U.S. Pat. No. 4,151,240 to Lucas teaches reheating and partially melting a web while the web is over the forming area of the forming screen. A melted polymer is desirable to form three-dimensional apertures since a melted polymer is more easily sucked into the apertures in a forming screen. Both U.S. Pat. No. 4,456,570 to Thomas and U.S. Pat. No. 4,151,240 to Lucas teach primarily using vacuum as a main source of pressure differential energy that is used for the work energy that changes a two dimensional web into a three-dimensional cell and causes an aperture to open in a film web. During the formation of a VFF, the polymer of the film typically undergoes a phase change from molten state in a flat form to a crystalline state in the new three-dimensional form.
In some cases, it is desirable to form textures on the lands of the VFF. To form textures on the lands of the vacuum formed film, lands are provided on the forming screen with textures provided thereon. The textures on the forming screen are then incorporated into the direct melt VFF film. Due to vacuum pressure, textures form on the lands of the subsequently formed VFF. As discussed above, the vacuum pressure differential also causes 3-D cells with apertures to be formed in the film.
The textures imparted on the VFF may be formed in a pattern. Examples of embossing patterns include straight lines, pyramids, diamonds, squares, and random matte. Further, more exotic patterns may be used including, exotic squiggly lines, spiral patterns, microscopic flower petals, and other ornamental designs.
A micropattern can also be incorporated into a precursor film by a reheat VFF process, via either cast embossing or blown embossing processes that are well known in the industry and that are discussed above. In a reheat process, external heat is applied to partially melt and form three-dimensional cells with apertures. Portions of the precursor film rest on the lands of the screen, which partially protects these portions of the precursor film from the heat. Therefore, only the portion of the film suspended over an opening of a cell in the forming screen is fully unprotected from exposure to heat. Thus, the suspended portion becomes melted and forms a three-dimensional cell with an aperture.
When a film layer is applied to a forming screen, the film layer typically has about 25 to 80 times less mass than a metallic screen mass beneath the film layer. Because of the mass ratio of the film layer to the screen, the screen acts as a “heat sink” in the land area where the precursor film is in intimate contact with the lands of the forming screen. The heat passes through the thin film and is absorbed by the screen such that no, or negligible, thermal distortion occurs on the land regions. As a result, any texturizing pattern embossed into the precursor film is maintained in the finished VFF.
Films produced by the methods above may be constructed of various materials having a selected mesh count, embossed thickness, a selected aperture pattern, a selected width of the lands or spaces between the apertures, and a selected pattern may be formed on the lands. The “mesh count” is the number of cells aligned in one inch of distance. Other variations may also be possible. Each configuration will exhibit distinct properties with respect to performance.
When measuring a VFF for percent open area, it is common to use any of the many computerized video devices that are available. The video camera, via magnification and contrast, can discern the openings from the lands and digitize the data to calculate the percent open area.
Unlike nonwoven material (NW), which exhibits capillary action for wicking fluids, formed films are made from polymer webs that do not transmit fluid unless the formed film is “formed” into a three-dimensional apertured sheet. Formed films may be tested for rewet. A lower rewet value is more desirable. Generally, preferred products have had a rewet value of less than one gram; i.e. a “fractional gram”. It has been found that products with a gram or more of rewet are typically viewed by consumers as being wet or damp in use.
Fluid acquisition rate is also critical to a functional topsheet. If the fluid acquisition rate is too slow, then a product using the topsheet may leak. The fluid acquisition rate is affected by several factors. The surface energy of the vacuum formed film is critical for fluid acquisition rate. Additionally, the fluid acquisition rate is directly correlated to open area. Additionally, the “loft”, or the required spatial distance between a fluid containing absorbent core and the skin of the user, must also have a certain measure to prevent a wetness factor of one gram or greater as exhibited by rewet values. Simply stated, if there are relatively large openings, as indicated by high % open area, and comparatively little separation space, as indicated by low loft, then fluid can overcome the short expanse of space through the center of the large opening, which results in reverse flow, or “rewet”.
Table 1, below, is derived from selected feminine napkin products from around the world that use a formed film coversheet. From the data in Table 1, the ratio correlation can be seen. From such data, the apparent line of separation of the loft to % open area ratio (L/OA Ratio) between a “dry” coversheet and “damp” coversheet would logically be about a L/OA Ratio of ≧10.
TABLE 1Open Area,Rewet,ProductLoft, μ%L/OA RatioGramsAlways55032.017.00.05Equate45528.516.00.15Siempre Libre45020.022.00.12Itimus37020.018.50.10CareFree (Euro)13025.05.24.85Magix10021.54.56.15Centre Libre19025.07.61.90
The term “rewet” implies that all of the fluid passes through the topsheet and then only the fluid coming back to the surface to “rewet” it is measured. However, with the many varieties of micro-embossing, crimping, and punching involved with these materials, often “wells” can be formed that trap fluid on the surface. The entrapped fluid accounts for about 15% of variation in the data. Also, as with any reliable test method, the method itself will have some variation of results, even within a given single material. This is offered to explain why the correlation is not exactly linear as, in theory, it should be.
Hole diameter is determined by the narrowest width of an aperture (specifically for aperturing oval or elliptical shapes) that can be determined as a function of mesh count and land width. From mesh count and land width, one can derive an approximate hole diameter, or span of the “supports” for the polymer sheet to be apertured.
A hole diameter of a typically known 60 mesh forming screen is usually no more than 200μ. Since a reasonable amount of metal must remain between holes in a forming screen (such that it will be robust enough to run in the VFF process), one can calculate hole diameter as follows. As explained above, “mesh” is the number of cells aligned in one inch of distance; hence, 1/60=0.017 inch center to center, or 430μ. One will need about 230 μ of metal land area to have a robust screen, leaving the nominal 200μ hole diameter for a 60 mesh pattern.
In addition to rewet performance and fluid acquisition performance during use, it has been found that the feel or tactile impression of a topsheet is important to consumers. Silk has been known for centuries to impart a unique and highly desirable tactile impression that has no other description than to say, “This feels silky”. The term “silky” alone provides enough description for average global consumers to grasp its meaning and recognize whether or not a product feels “silky” or merely soft and cloth like. In repeated blind panel tests various fabrics such as felt, flannel, cotton diapers, polyester/cotton clothing fabric, wool, and silk were tested. The panels easily discern a silky tactile impression (STI) of silk cloth over the other cloth materials.
For many years, the feminine napkin market has been segmented into women who prefer a nonwoven coversheet and women who prefer a film coversheet. The market segmentation is particularly found in westernized countries. Those who prefer the nonwoven type seem to prefer the cloth like tactile impression and the perceived “comfort” that they derive from it.
Users of the nonwoven type, however, sacrifice the dry cleanliness of the VFF type. Nonwovens have capillarity due to having their many fibers in close proximity to the absorbent core. Capillarity is good for transmitting fluid through a coversheet via the capillary action of the nonwoven. Unfortunately, “wicking” by capillary action can also act in reverse. Therefore, nonwovens are not known for providing good rewet values. A good rewet value is indicative of dry cleanliness during use.
Those who prefer the film type seem to prefer the improved cleanliness and anti-rewet, particularly that of the VFFs. Many VFF coversheets have large openings which readily accept semi-coagulated matter found in menses. VFFs also provide the afore-mentioned prevention of the fluid rewetting to the top plane of the film. The prevention of rewet is derived from the superior loft of the VFF material. Hence, those who prefer the prior art film type forgo a bit of cloth like tactile impression derived from the presence of fibers of a nonwoven to achieve the cleanliness, which is especially true of a VFF. A film that delivers the perceived comfort of a nonwoven with the improved cleanliness and anti-rewet is desirable. Therefore, much effort has been made in attempts to derive the benefits of both types, some with market success; however, no VFF to date has delivered both the cleanliness and a silky tactile impression.